1. Field of the Invention
This invention concerns pressure transducer arrangements in which an electrical signal is utilized to produce a corresponding fluid pressure level and the application of such a pressure transducer to an automatic temperature control system for automotive passenger compartment temperature controls.
2. Description of the Prior Art
Automatic temperature control systems of the type utilized in modern day automotive vehicles commonly use a combined air cooling and heating of circulated air in order to maintain temperature levels within the passenger compartment at selected levels. These systems typically use air ducting through which an air flow is directed by means of a blower, the air flow passing first over an air conditioner evaporator coil to initially cool the air mass flowing through the duct, the air flow then being directed to a diverter box which proportions the air flow between a bypass duct passage and a heater passge in which is disposed the heater core. A variably positioned blend air door proportions the quantity of air passing into either the bypass duct or the heater duct so as to control the temperature of the air mass flowing through the ducting, the air flow then passing into the passenger compartment.
The temperature control is carried out by means of a manual temperature selector device such as a rheostat and a passenger compartment temperature sensor such as a thermistor, both of which produce electrical signals corresponding to the desired temperature level and the sensed temperature level, respectively. Upon development of a temperature error signal indicating a difference between the selected temperature and the actual temperature, an electrical error signal is produced as by a differential amplifier, the error signal having a sense corresponding to the direction of the error, i.e., either above or below the manually selected temperature level. This electrical error signal is typically applied to a vacuum modulator device which generates a vacuum pressure level corresponding to the error signal which vacuum pressure is utilized to properly position the blend air door by means of a vacuum operated actuator. The system variables are calibrated such that the change in position of the blend air door increases or decreases the temperature of the air flow within the ducting system so as to correct the temperature level in the passenger compartment.
That is, for a relatively great sensed temperature level above the selected temperature level, the vacuum pressure is adjusted to cause the blend air door to assume a position whereby most or all of the air flow is directed through the bypass duct and maximum cool air is circulated into the passenger compartment. Conversely, if the sensed temperature level is below that selected, the blend air door position is adjusted to increase the air flow past the heater core to increase the temperature of the air passing into the passenger compartment to correct the temperature differential.
The degree of the error signal in either sense determines the extent of movement of the blend air door in the direction tending to correct the temperature difference condition.
In some conventional prior art systems, the modulator device utilizes a movable member which generates a force corresponding to the electrical error signal, which acts against a pressure responsive diaphragm member such as to create a force stabilized pressure regulator. These arrangements produce a pressure corresponding to the magnitude of the error signal typically by use of a metering valve which opens to allow the diaphragm chamber to be in communication with a low pressure source such as the engine manifold. Such a system is disclosed in U.S. Pat. No. 3,877,638 to Amano et al.
A difficulty with such an approach is that the pressure levels achieved demonstrate considerable variations for variations in the vacuum source pressure. Also, the electrical error signal must generate a force which is not inconsiderable to balance the force generated by the change in pressure within the modulator, to slow the system response due to the thermal inertia of the parts involved if thermal effects are utilized. The response to the system is further slowed since the approach inherently involves the controlled communication of the low pressure source within the enclosure within which the pressure level is to be regulated such that changes in pressure to the actuator could not be changed by applying full vacuum.
Such a force stabilized system has been deemed by those working in the art to be necessary since the use of a temperature feedback to control the stabilization of the control loop was impossible to stabilize due to the long time lag between the movement of the blend air door and the achieving of the reduction or increase in the temperature level of the passenger compartment such that as a practical matter such control systems could not be stabilized with a thermal feedback. Accordingly, the error signal had to be stabilized directly by means of a force produced by the change in pressure or some such similar solution.
In the Weaver et at Pat. No. RE 27,699, an improvement to such systems is disclosed in that a pressure transducer device is utilized as a vacuum modulator in which the vacuum modulator does not involve the use of large pressure created forces. Rather, a movable core is moved linearly with respect to the error signal and free communication with an interior space within which the pressure is to be developed is created to provide a very rapid generation of the new corresponding pressure level upon development of an error signal. This system is stabilized with a position feedback potentiometer associated with the blend air door which generates an electrical signal corresponding to the desired position of the blend air door in correspondence with the sense and degree of the error signal. The system has proved to be highly satisfactory inasmuch as the response of the system is much improved and quickly stabilizes the modulated vacuum pressure. This new vacuum pressure level is very quickly achieved since open communication between the low pressure source and the interior of the modulator is distinguished from the approach typified by the Amano et al patent mentioned above.
However, the pressure regulator device shown in the Weaver et al patent, while highly satisfactory in performance, suffers from its relative complexity and high cost of manufacture, which element adds significantly to the cost of the system.
In the Amano et al patent, there is disclosed the use of a heated bimetal to produce a force which corresponds to the electrical error signal. As described above, this basis system has certain drawbacks due to the force balance principle involved, in which relatively high force levels must be generated by the bimetal elements. However, a heated bimetal device does produce a convenient and effective way of converting an electrical signal into mechanical movement and could be utilized in a system such as the Weaver et al patent to provide a simple communication of a pressure or vacuum port with an interior enclosure. The difficulty in achieving such an application for this device would be in producing a satisfactory response from the heated bimetal without creating the danger of incidental overheating of the bimetal and/or erratic performance thereof. That is, if relatively large heating currents were utilized over a period of time in order to quickly heat the bimetal element to enhance its responsiveness, such large currents would be difficult to properly relate to the specifics of the bimetal element without destroying or degrading the element. On the other hand, if lower currents were utilized, excessive time periods of response would likely be encountered and the slope of the time-deflection curve would be too shallow for adequate response particularly for such applications as described above.
Also, the relationship between the heating circuit current and the bimetal would be difficult to design if such design was constrained by the steady state or continuous operation temperature levels achieved by the heating circuit. Furthermore, the larger deflections associated with large temperature current levels would reduce the return response of the bimetal arm since the deflection would be relatively gross and temperature levels high and considerable time period would be required in order to cool the bimetal to a point where it would again seat on the control port. These difficulties would prevent the application of such simple heated bimetal elements to applications such as the vacuum modulator device shown in the above-referenced Weaver et al patent.
Accordingly, it is an object of the present invention to provide a heated bimetal actuator device for application as a pressure or vacuum port communication control which the response movement of the bimetal both in the opening and closing movements are very rapid.
It is yet another object of the present invention to provide a pressure transducing arrangement utilizing such heated bimetal which generates a pressure level corresponding to an electrical signal.
It is still another object of the present invention to provide such a pressure transducing arrangement suitable for use as a vacuum modulator device in which intermittent communication of an interior chamber with a vacuum source is carried out in order to control and generate a controlled vacuum pressure level within the chamber.
It is still another object of the present invention to provide a pressure transducing arrangement incorporated in a vacuum modulation device combined with an automatic temperature control system of the type described.